To get in line with the global trend of environmental protection and eco-friendly regulations, electronic product manufacturers nowadays are developing and manufacturing halogen-free electronic products. Advanced countries and electronic manufacturing giants set forth schedules of launching mass production of halogen-free electronic products. As a result of the promulgation of the Restriction of Hazardous Substances (RoHS) by the European Union, hazardous substances, such as lead, cadmium, mercury, hexavalent chromium, poly-brominated biphenyl (PBB), and poly-brominated diphenyl ether (PBDE), are strictly prohibited from being used in manufacturing electronic products or their parts and components. A printed circuit board (PCB) is an indispensable and fundamental basis of the semiconductor industry and electronic industry; hence, printed circuit boards bore the brunt of international halogen-free regulations when international organizations set forth strict requirements of the halogen content of printed circuit boards. For example, the International Electrotechnical Commission (IEC) 61249-2-21 requires that bromide content and chloride content shall be less than 900 ppm and the total halogen content shall be less than 1500 ppm. The Japan Electronics Packaging and Circuits Association (JPCA) requires that both bromide content and chloride content shall be less than 900 ppm. To enforce its green policies, Greenpeace calls on manufacturers worldwide to get rid of polyvinyl chloride (PVC) and brominated flame retardants (BFRs) from their electronic products in order to conform with the lead-free and halogen-free requirements of green electronics. Hence, the industrial sector nowadays is interested in rendering related materials halogen-free and sees this technique as one of its key research topics.
Electronic products nowadays have the trend toward compactness and high-frequency transmission; hence, circuit boards nowadays typically feature a high-density layout and increasingly strict material requirements. To mount high-frequency electronic components on a circuit board, it is necessary that the substrate of the circuit board is made of a material of a low dielectric constant (Dk) and dielectric dissipation factor (DO in order to maintain the transmission speed and the integrity of a signal transmitted. To allow the electronic components to function well at a high temperature and a high-humidity environment, it is necessary for the circuit board to be heat resistant, fire resistant, and of low hygroscopicity. Epoxy resin is adhesive, heat resistant, and malleable and thus is widely applicable to encapsulants and copper clad laminates (CCL) of electronic components and machinery. From the perspective of fire prevention, epoxy resin is incapable of flame retardation, and thus epoxy resin has to acquire flame retardation capability by including a flame retardant therein. For example, a halogen, such as bromine, is included in epoxy resin to not only bring about flame retardation capability thereof but also enhance epoxy reactivity. Furthermore, after long use, halides are likely to decompose at high temperature, which often results in corrosion of fine circuits. Also, upon their combustion, discarded electronic parts and components produce halides which are most hazardous and environmentally unfriendly. To find an alternative to the aforesaid halide-based flame retardant, researchers attempt to use a phosphorous compound as a flame retardant, for example, adding phosphate ester to an epoxy resin composition.
A conventional circuit board manufacturing method, such as a conventional method of manufacturing a copper-clad substrate (also known as copper clad laminate, CCL), involves heating and combining a reinforcement material (such as a glass fiber fabric) and a thermosetting resin composition made of an epoxy resin and a curing agent to form a prepreg, and then laminating the prepreg and the upper and lower copper foils together at a high temperature and a high pressure. The prior art usually teaches using a thermosetting resin composed of an epoxy resin and a hydroxyl-containing phenol novolac resin curing agent. Due to the combination of the phenol novolac resin and the epoxy resin, epoxide ring-opening reactions end up with another hydroxyl which not only increases the dielectric constant and the dielectric dissipation factor inherently, but also reacts with water readily and thereby renders the thermosetting resin more hygroscopic.
Glass transition temperature (Tg) is the most important index to the heat resistance of copper clad laminates. Glass transition temperature (Tg) increases with the heat resistance of copper clad laminates. The degree of cure of copper clad laminates is denoted with delta Tg (where delta Tg=Tg2−Tg1), which is measured with an apparatus, such as DSC, TMA, or DMA. A small delta Tg value indicates that the copper clad laminates are completely cured. Conversely, a large delta Tg value indicates that the copper clad laminates are incompletely cured or are unlikely to be completely cured. When incompletely cured, the copper clad laminates manifest high hygroscopicity, low heat resistance, and unstable high-frequency dielectric property (Dk or DO to the detriment of the copper clad laminates.
Taiwan Patent 1311568 discloses a phosphorous-containing epoxy resin composition comprising ingredients: (A) a phosphorous-containing epoxy resin; (B) a curing agent; (C) one or more epoxy resin; and (D) an inorganic filler. The ingredient (B), the curing agent, comprises a benzoxazine (BZ) resin and a styrene maleic anhydride (SMA) copolymer. However, copper clad laminates manufactured in accordance with the aforesaid resin composition are flawed with an overly large delta Tg value. In general, those copper clad laminates which are manufactured in accordance with the aforesaid resin composition using the curing agent comprising a benzoxazine (BZ) resin and a styrene maleic anhydride (SMA) copolymer have a delta Tg value larger than or equal to 10° C.
Accordingly, it is important for printed circuit board material suppliers to develop materials which are halogen-free and have a low delta Tg value, and apply the materials to printed circuit board manufacturing.